Mineral acid solutions are widely used at Department of Defense (DoD) facilities where metal-finishing operations are performed as part of depot- and intermediate-level maintenance. During use, the acid is consumed and weakened, and is also rendered less effective by increasing concentration of solubilized metals. This project demonstrated that by separating the metals and recycling the acid bath through diffusion dialysis, the waste stream can be recovered and reused. Diffusion Dialysis (DD) is an ion-charge selective membrane separation process, which can separate acids from waste solutions. Acids pass through the membrane and are recovered for re-use while metals are rejected. Two demonstrations of full-scale DD units were conducted: (1) a stand-alone, batch-mode unit at the Tobyhanna Army Depot, Pennsylvania for treating spent copper bright dip (CBD) and spent magnesium bright dip (MBD), and (2) a dedicated, continuous-mode unit operated for 22 weeks at the Rock Island Arsenal, Illinois for recycling chrome-stripping solution.
DD was successfully demonstrated as a technically feasible process. For batch operation, projections showed consumption of CBD and MBD would be reduced by as much as 50% and 40% respectively, although bath service life would be 30-35% shorter because the acid recovered by DD for reuse was slightly lower quality than fresh acid. Acid recovery was 70-85% and rejection of metals was 60-85%. Results for continuous operation on the chrome-stripping bath were similar except that rejection of metals forming chloride complexes was reduced to 15-40%. However, these metals were not predominant contaminants and bath performance was successfully maintained.
The DD process is simple, reliable, and easy to install and operate. Its implementation would enable the DoD to significantly reduce acid procurement and hazardous waste disposal costs associated with conventional metal-finishing operations. The cost-effectiveness of a DD installation depends on the value of the acid recovered, the feasibility of treating the metal-laden by-product stream in an on-site industrial wastewater treatment plant (IWTP) and on the scale of the metal-finishing operation. For higher-value acids such as nitric acid, sulfuric acid, and hydrofluoric acid recovered in the CBD/MBD batch operation at Tobyhanna, payback period could be less than 2 years for a $22K capital cost if large-size batches were available for processing. A minimum production rate of 1,500 gallons per year spent acid appears likely for an acceptable payback. For low-value hydrochloric acid, the process is unlikely to be attractive at any scale of operation. Cost analysis showed that the payback period for the continuous system at Rock Island would be 8-9 years based on a $32K capital cost for the DD installation.
DD was a technically feasible means of recovery of acids from the three metal-finishing operations investigated. However, its impact on bath service life and associated life cycle cost savings for other types of electroplating operations would have to be determined on a case-by-case basis. Reductions in acid purchases of 30-70% are expected. Also, cost-effective installation of DD may be precluded at facilities that do not have an on-site IWTP.